The present invention relates to a form of non-volatile memory in integrated circuits which can be programmed before or after encapsulation in any type of package. More particularly, the present invention relates to the use of on-chip circuitry for blowing fuses for precision trimming analog integrated circuits encapsulated in plastic package devices.
It is well known in the art of integrated circuit design to use one or more fuses, in conjunction with other fuses and circuit elements, to set a circuit parameter such as a voltage, a current, or a voltage or current gain. Fuses generally have an intact, or uninterrupted, state and a blown, or interrupted, state. In the intact state, the fuse is a low resistance element capable of conducting current. In the blown state, the fuse has a very high resistance and generally does not conduct current. When used in conjunction with other circuit elements to set a circuit parameter, the fuse may be configured so that its intact state corresponds to a first value of the circuit parameter, and its blown state corresponds to a second value of the circuit parameter.
Fuse trimming circuits are commonly used in analog integrated circuit devices. Such analog integrated circuits may include digital signal processors, telecommunications devices, and operational amplifiers. In analog integrated circuits, it is often desirable to precisely trim circuit parameters to a predetermined range of values. While analog integrated circuits may be designed to produce such parameters within a certain range of values, variations in manufacturing processes may produce an unacceptably wide tolerance level for parameters after manufacturing. Accordingly, it is well known to use fuse trimming to improve the tolerance range of analog integrated circuit parameters for high performance applications.
In prior art devices, fuses have been fabricated on the surface of an integrated circuit device directly adjacent to and in electrical connection with a large area of metal known as a probe pad. The fuse is fabricated from one of the conductor layers used in the integrated circuit device, such as a metal layer or a low-resistance polysilicon layer. As is well known in the art, the fuse is formed of a narrow strip of the conductor layer. When the current through the fuse exceeds a predetermined amount, the fuse "blows" by interrupting the strip of conductor layer. The final passivation layers of the integrated circuit device must be removed from the probe pad to allow direct contact with the probe pad by a probe.
As noted, a first end of the fuse is in electrical contact with the probe pad, and a second end of the fuse is electrically connected to another node on the integrated circuit device, such as ground. In such prior art devices, the first end of the fuse is also connected through a resistor to a second circuit node, such as the most positive potential in the circuit, or VCC. This first end of the fuse is also connected to circuitry for sensing the voltage present at the first end of the fuse and generating a desired circuit parameter. With the low resistance fuse intact, and with power supplied to the integrated circuit, the first end of the fuse will be pulled to the same potential as the second end of the fuse, or ground. Current will flow through the resistor from VCC, causing a potential drop across the resistor.
In response to the VCC level present at the first end of the fuse, the circuitry coupled to the first end will generate a circuit parameter such as a current or voltage having a first value. When it is determined that this first value is outside of a predetermined range of values, the fuse will be blown. To blow the fuse, a probe contacts the probe pad and supplies sufficient voltage and current across the fuse to cause the fuse to blow, converting from its intact state to its blown state. With the fuse in its blown state, no current flows through the fuse, and the resistor serves as a pull-up resistor, pulling the potential at the first end of the fuse to VCC. When the circuitry coupled to the first end of the fuse detects the VCC potential, the circuitry produces the circuit parameter at a second value.
Because of the need to contact the probe pad with a probe, fuse trimming by blowing one or more fuses must occur at the wafer sort step in the manufacturing process for the integrated circuit device. At wafer sort, the wafers containing the completed integrated circuit devices are contacted with probes to determine functionality of each of the integrated circuit devices. Nonfunctional devices are marked and rejected. Also at wafer sort, predetermined circuit parameters, such as voltages and currents, are measured. In response to the measured circuit parameter, and in accordance with a predetermined program of instructions, one or more fuses on the integrated circuit device may be blown to shift the measured circuit parameter to a preferred value. After wafer sort, nonfunctional chips are discarded and functional chips are packaged for further testing.
In many applications, it is preferable to package integrated circuit devices in plastic packages. Plastic packages are lighter, less expensive and less bulky than alternative packages, such as ceramic packages. However, the manufacturing process by which an integrated circuit device is encapsulated in a plastic package causes circuit parameters to change significantly in the completed, packaged device. The plastic packaging process causes plastic stress, including mechanical and thermal stresses, which varies circuit parameters. From statistical samples taken from a large number of integrated circuit devices, both before and after plastic packaging, it is known that both the mean and the standard deviation of the statistical distribution of the measured circuit parameters will vary as a result of the plastic packaging process.
One solution to this shift in the statistical distribution of measured parameters has been reverse trimming. Where a parameter, such as a voltage gain, is known to shift by a predetermined amount as a result of plastic packaging, the parameter can be reverse trimmed at wafer sort to accommodate the known shift due to plastic packaging.
However, reverse trimming also has its limitations. First, the statistical distribution shift due to plastic packaging is not always constant. As a result, circuit parameters may be overtrimmed or undertrimmed, either of which can cause yield loss. Also, the standard deviation in the statistical distribution of circuit parameters increases following plastic stress due to the plastic packaging process, also causing yield loss.
A further problem encountered during fuse trimming results from marginally-blown fuses. A fuse in its intact state has a very low resistance, such as 200 ohms. A normally-blown fuse has a very high resistance, such as 20 megohms. These values are sufficiently different that associated circuitry can readily discriminate the intact state of the fuse from the blown state of the fuse. However, it is known that some fuses will not fully blow, but assume a marginally-blown state where the fuse resistance is on the order of 10K ohms. The marginally-blown state may result from one or more remaining filaments of fuse material.
Marginally-blown fuses are problematical since it is known that marginally-blown fuses may permit fuse re-growth to occur. In prior art fuse trimming circuits, as described above, a voltage is maintained across the blown fuse during normal operation of the integrated circuit. The pull-up resistor couples the first end of the fuse to VCC, and the second end of the fuse remains at ground potential. Where the fuse is marginally blown, the presence of this potential difference is known to cause the fuse to grow back and reassume a low resistance, intact state. As a result of this fuse regrowth, the circuitry which detects the intact or blown state of the fuse may incorrectly detect an intact state for a regrown fuse, causing a circuit parameter to be set to an incorrect value.
Accordingly, it is an advantage of the present invention to provide fuse trimming in integrated circuit devices which have been packaged in plastic integrated circuit packages. However, the invention can also support fuse trimming before encapsulation just as easily.
It is a further advantage of the present invention to provide for blowing on-chip fuses using only on-chip circuitry.
It is a still further advantage of the present invention to eliminate inaccuracies in circuit parameters generated by fuse trimming circuits by eliminating fuse regrowth.
It is a still further advantage of the present invention to test for marginally-blown fuses in order that integrated circuit devices having such marginally-blown fuses can be rejected as not fully functional devices.
Further advantages and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the preferred embodiment of the present invention.